The present invention relates to a low or high speed printer with a carriage which carries a printing unit and is moved laterally along a printing paper.
The use of a combination of a pulley and a wire rope has been known as a mechanism for controlling a movement of the carriage to move it laterally along the printing paper at high speed and to stop it exactly at predetermined positions. The driving mechanism of the type mentioned above for converting a rotational movement of a motor into a straight movement of the carriage has many advantages over the conventional system such as a lead screw driving system, a steel belt driving system or a timing belt driving system in that the construction is much simpler, the moment of inertia is very small, the durability and reliability are very high and the accumulation of errors due to a variation of the feeding pitch is relatively small.
There is a known carriage driving mechanism for a printer having a wire rope means, which comprises a pair of wire rope segments of a same length, one end of each segment being attached to a driving pulley and another end fixed to the carriage through a stationary pulley in the frame.
U.S. Pat. No. 3,872,960 discloses a typical example of the pulley and wire rope driving mechanism for a carriage of a high speed printer, in which the carriage carrying a printing wheel is coupled to a driving motor through a pulley which has attached to it each end of left and right wire rope segments of equal length, the other ends of the wire rope segments being attached to the machine frame. The wire rope segments are coupled to the carriage through a pulley arrangement.
In these prior printers, the driving force for driving the carriage is transmitted through the single wire rope and this wire rope has to be stretched under a certain stress in order to prevent the wire rope from being late in moving the carriage due to the elongation of the wire rope and to thereby insure an exact transmission of the driving force of the motor to the carriage. Therefore, the thickness of the wire rope should be large and hence the diameter of the pulley should also be large enough correspondingly for maintaining the resistance to the repeat bending fatigue, resulting in an increase of the movement of inertia.
The above problems inherent to the prior printer, may be resolved by, for example, utilizing a pulley block to reduce the tension force to be exerted on the wire rope and making the diameter of the driving pulley correspondingly larger to take up the wire rope increased in length due to the use of the pulley block mechanism.
In this case, however, since the rotational angle of the motor may be increased in comparison with the system which does not use the pulley block and the inertia of the carriage unit converted on the motor shaft may be increased due to the increase of the diameter of the driving pulley, the tension force exerted on the wire rope is increased in spite of the intended reduction thereof due to the utilization of the pulley block. Therefore, it is very difficult to select an optimum condition between the various elements.